Fugitive emissions result from releases of airborne matter to the atmosphere from diffuse sources, which can include landfills, reservoirs, effluent ponds, mines, natural deposits, or even a collection of point-sources such as cities, industrial plants, or a herd of animals. Fugitive emissions can also include emissions from point sources, such as smokestacks, flares, wells, exhaust tubes, leaks and vent pipes, that have been released to the atmosphere. The airborne matters can be greenhouse gases, gaseous organic compounds, polluting gases, or particulate matter. The atmospheric volume within which the airborne matters exist is referred to as a plume and the emission flux is the flow rate of the airborne matter.
Flux boxes, dynamic closed chambers, and micrometeorological methods are point sampling techniques from which the emission flux from an area source must be extrapolated. Due to the extent and non-homogeneous nature of many area sources, assessment of fugitive emissions using traditional point sampling techniques can be problematic (Thoma, 2008). The accuracy of the flux box and dynamic closed chamber methods are dependent on the number of flux box or chamber tests conducted and provide an average flux over the sampling period. Use of flux boxes and dynamic closed chambers can also be time consuming and are not applicable to sources such as reservoirs or mines. Field tests for a large area can require many days to complete. If the fugitive emissions are dominated by one or more concentrated sources, such as cracks in a landfill, these methods may not be suitable.
Micrometeorological methods are applicable at locations that are uniformly flat and are a couple of hundred meters from the crest of a slope (Scharff, 2005). Thus the method is not applicable to many sites, which are sloping or have varying topography.
Atmospheric tracer methods involve releasing a tracer gas, often sulphur hexafluoride or nitrous oxide (potent greenhouse gases), or acetylene (Czepiel, 1996) into the emission plume. This method is restricted to situations where the source is sufficiently strong such that it can be measured at a sufficient distance downwind where adequate mixing of the airborne matter and tracer gas has occurred. As such, it is not suitable for confirmation of emissions from sites where emission rates are low (Czepiel, 1996). Measurement of flow of the tracer gas and physical sampling of the downwind air more than 100 m from the source is required (Czepiel, 1996; Griffith, 2008). Two to three different gas analyzers are required. For large area sources, such as cities, the method was reported to have an error as large as 65% for measured flux (Lamb, 1995).
The dynamic plume or inverse modelling method uses a fast response airborne matter detector to obtain concentration measurements in the plume, and then a dispersion model is used to estimate emissions (Hensen, 2001). A slight variant of this method is to use a flame ionization detector to obtain airborne matter concentrations just above the emission surface, and then to use an air dispersion model to calculate an emission rate (Huitric, 2006). The accuracy of the dynamic plume method is, in part, dependent on the accuracy of the dispersion model. Dispersion models can be complex and incorporate many simplifying assumptions.
Compared with point sampling techniques, optical remote sensing instruments have the advantage of sampling over a large volume, along an open path, and are able to provide continuous, real-time measurement of the integrated concentration of airborne matter. U.S. Pat. No. 6,542,242 teaches a method for mapping of airborne matter using path-integrated optical remote sensing (ORS) with a non-overlapping variable path beam length geometry (Radial Plume Mapping). Radial Plume Mapping uses optical remote sensing instruments to obtain path integrated data, that is processed reiteratively using a cumulative distribution function to provide a simplified map of the concentration of airborne matters. The assumed radial concentration pattern is determined based on an assumed cumulative density function. The method, in a vertical configuration, requires a ground-based, stable vertical structure on which to mount reflectors.
The United States Environmental Protection Agency's Other Test Method 10 (OTM 10) describes a method of applying the Radial Plume Mapping methodology in a vertical configuration for the measurement of fugitive emission flux. The method has been validated for application to relatively small, isolated area sources. Efforts to apply it to large area sources with complex topography are being attempted. However, this method may not sample the entire plume volume, as the height of the measurement path is limited by the angle from which the ground-based instrument is pivoted in order to target the highest vertical reflector (which is ground based). If the upper limit of the emission plume is higher than the highest measurement path, or the measurement beam paths do not bracket the emission plume, the accuracy of the method can significantly decrease as shown by the tracer release results in Thoma (2008).
The conventional mass balance method involves measuring the wind and airborne matter concentration profiles through the full height of the boundary layer containing emissions from the emitting source, and integrating the concentration and wind speed with respect to the height above ground surface. The method uses a ground-based mast (Tregoures, 1999) or a tethered balloon with a sampling sonde for point sampling of the air. The sampling balloon is held at different heights to obtain variations of concentration with elevation (Lamb, 1995). This point sampling method introduces an error since the whole region is not sampled.
U.S. Pat. No. 4,135,092 and U.S. Pat. No. 4,204,121 teach mass balance methods using either a number of point samplers mounted on a vertical pole or line, an aircraft flying through the plume at various elevations collecting samples at several height intervals, or vertically spaced infra-red radiation transmitters on a mast opposite another mast with a matching series of infra-red receptors. Sampling is only along horizontal lines and there is no teaching on the use of optical remote sensing instruments with targets or reflectors. Sampling can be made upwind of the source area to evaluate the contribution of incoming pollution to the apparent fugitive emission rate. However, it does not teach how to account for a natural background concentration of a pollutant in the atmosphere.
U.S. Pat. No. 6,864,983 teaches the use of a spectrometer for receiving absorption spectra from the sun, from which emission flux can be calculated. The method depends on the availability of direct sunlight and may only be used on sunny days. In addition, the accuracy of the method for some gases is questionable due to the long absorption distance through the atmosphere. For example, the significant background concentration of methane in the atmosphere results in a very large integrated concentration of methane, compared with the contribution of most methane emission plumes.
Mapping of airborne matters can also be carried out using Differential Absorption Laser Detection and Ranging (DIAL). It can be classified as a mass balance method that uses two Nd:YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) lasers. This equipment can map the concentration of airborne matters in the air, from which an emission flux can be calculated (Chambers, 2006). In an emission flux measurement application, this equipment is ground based, expensive, heavy and bulky.
U.S. Pat. No. 6,822,742 and U.S. Pat. No. 6,995,846 provide an airborne DIAL, using ND:YLF (neodymium-doped yttrium lithium fluoride; Nd:YLiF4) lasers for detection of natural gas pipeline leaks, providing a path-integrated concentration of methane and ethane. Unlike the above method that uses two Nd:YAG lasers, this DIAL instrument does not map the concentration of airborne matter in the air. There is no teaching of measuring or quantifying emission flux of the gas leak.
A helicopter-borne spectroscopy method “Airborne Laser Methane Assessment” (ALMA) using a tunable diode laser (TDL) measures a path-integrated concentration in ppm-m on a vertical line. However, Babilotte (2008) state that ALMA “provides a path-integrated concentration on a vertical line, and does not allow fluxes quantification”.
All of the above methods either have significant constraints that limit their applicability and accuracy, or are tools for which a methodology to measure fugitive emission fluxes is not available. What is needed is a method that can obtain data within a reasonable time frame, can carry out measurements throughout the entire thickness and width of the emission plume, thereby improving accuracy, does not involve very limited point sampling of airborne matter concentrations, does not require infra-red transmitters to be opposite infra-red receivers, and is suitable for application to emission sources over a large area or of a plume height that extends above ground-based moveable platforms. Furthermore, the method may not require access more than 100 m downwind of a source, does not require complex numerical modelling of airborne matter dispersion or mapping of airborne matter concentrations, and does not require specific release of a tracer gas.